The human epidermal growth factor receptor 3 (HER3, also known as Erbb3) is a receptor protein tyrosine and belongs to the epidermal growth factor receptor (EGFR) EGFR/HER subfamily of receptor protein tyrosine kinases (RTK), consisting of EGFR (HER1/Erbb1), HER2/Erbb2, HER3/Erbb3 and HER4/Erbb4. EGFR and HER2 are among the most well-established oncogenic RTKs driving the tumorigenesis of multiple types of solid tumors, including major categories such as breast, colorectal, and lung cancers. The tyrosine kinase activities of EGFR and HER2 have been shown to be essential for their oncogenic activities.
Like the prototypical EGFR, the transmembrane receptor HER3 consists of an extracellular ligand-binding domain (ECD), a dimerization domain within the ECD, an transmembrane domain, and intracellular protein tyrosine kinase domain (TKD) and a C-terminal phosphorylation domain (see, e.g., Kim et al. (1998), Biochem. J. 334, 189-195; Roepstorff et al. (2008) Histochem. Cell Biol. 129, 563-578).
The ligand Heregulin (HRG) binds to the extracellular domain of HER3 and activates the receptor-mediated signaling pathway by promoting dimerization with other EGFR family members (e.g., other HER receptors) and transphosphorylation of its intracellular domain. HER3 has been shown to lack detectable tyrosine kinase activity, likely due to a non-conservative replacement of certain key residues in the tyrosine kinase domain. Therefore, a consequence of this kinase-deficiency, HER3 needs to form hetero-dimers with other RTKs, especially EGFR and HER2, to undergo phosphorylation and be functionally active.
The central role for HER3 in oncogenesis is acting as a scaffolding protein to enable the maximum induction of the PI3K/AKT pathway. HER3 has been shown to contain a cluster of six C-terminal tyrosine-containing motifs that when phosphorylated, mimics the consensus PI3K/p85 binding site. Hence by forming heterodimers with HER3, the upstream onco-drivers, EGFR, HER2, cMET and FGFR2, can couple most efficiently to the PI3K/AKT pathway. Therefore, it is reasonable to expect that a loss of HER3 activity can block cancer progression in diverse systems driven by divergent RTKs. Studies have shown that HER3 siRNA knockdown in HER2-amplified breast cancer cells led to similar anti-proliferation effects as HER2 siRNA knockdown, further demonstrating the cancer's critical need for HER3.
Besides promoting tumor growth in unstressed conditions, HER3 has been found to be highly involved in conferring therapeutic resistances to many targeted drugs, including EGFR tyrosine kinase inhibitors, HER2 monoclonal antibodies such as trastuzumab, as well as small molecule inhibitors of PI3K or AKT or MEK. This adds another layer of attraction to HER3 as a promising cancer target for both primary tumor debulking as well as combating cancer resistance issues that invariably come up despite initial clinical responses.
HER3 has two different ways to dimerize with its partner RTKs: ligand-dependent (in the presence of HRG) or ligand-independent. In terms of HER2-HER3 dimers, it is known that in cells with low to medium HER2 expression, HER3 can only complex with HER2 after ligand-binding; in contrast, in cells with amplified HER2 (HER2 IHC 3+), they form spontaneous dimers without HRG (Junttila et al. (2009) Cancer Cell. 15(5):429-40). The dimers formed in the presence or absence of the ligand are structurally distinct as was demonstrated by an earlier study showing that trastuzumab/Herceptin® (Genentech/Roche HER2 monoclonal antibody approved for HER2 3+ breast cancers) can only disrupt the ligand-independent dimer but not the ligand-dependent dimer, whereas pertuzumab\Omnitarg® (rhuMAb 2C4, Genentech/Roche HER2 monoclonal antibody in phase 3 trials) can only disrupt the ligand-dependent dimers.
Dimer formation between HER family members expands the signaling potential of HER3 and is a means not only for signal diversification but also for signal amplification. HER3 has been shown to be phosphorylated in a variety of cellular contexts. For example, HER3 is constitutively phosphorylated on tyrosine residues in a subset of human breast cancer cells overexpressing HER3 (see, e.g., Kraus et al. (1993) Proc. Natl. Acad. Sci. USA 90, 2900-2904; Kim et al. (1998), Biochem. J. 334, 189-195; Schaefer et al. (2004) Cancer Res. 64, 3395-3405; Schaefer et al. (2006) Neoplasia 8, 612-622). Accordingly, therapies that effectively interfere with HER3 phosphorylation are desirable.
In addition, HER3 has been found to be overexpressed and/or overactivated in several types of cancers such as breast cancer, ovarian cancer, prostate cancer, liver cancer, kidney and urinary bladder cancers, pancreatic cancers, brain cancers, hematopoietic neoplasms, retinoblastomas, melanomas, colorectal cancers, gastric cancers, head and neck cancers, lung cancer, etc. (see, e.g., Sithanandam & Anderson (2008) Cancer Gene Ther. 15, 413-448). In general, HER3 is frequently activated in EGFR, HER2, C-Met, and FGFRII-expressing cancers.
A correlation between the expression of HER2/HER3 and the progression from a non-invasive to an invasive stage has been shown (Alimandi et al., Oncogene 10, 1813-1821; DeFazio et al., Cancer 87, 487-498; Naidu et al., Br. J. Cancer 78, 1385-1390). Thus, HER3 can be used as a diagnostic marker for increased tumor aggressiveness and poor survival. Sustained HER3 activation of PI3K/AKT has been repetitively shown to account for tumor resistance to EGFR/HER2 inhibitors.
Although the role of HER3 in the development and progression of cancer has been explored (see, e.g., Horst et al. (2005) Int. J. Cancer 115, 519-527; Xue et al. (2006) Cancer Res. 66, 1418-1426), HER3 remains largely unappreciated as a target for clinical intervention. Most current immunotherapies primarily focus on inhibiting the action of HER2 and, in particular, heterodimerization of HER2/HER3 complexes (see, e.g., Sliwkowski et al. (1994) J. Biol. Chem. 269, 14661-14665). Thus, it is an object of the present invention to provide improved immunotherapeutic agents that effectively inhibit HER3-mediated cell signaling that can be used for diagnosis, prognosis prediction, and treatment of a variety of cancers.